BBC Washington Correspondent Tom Carver: "The aim of the radar in question is to detect the undetectable"

Defence Minister John Spellar spoke after reports that the theft from the Los Alamos laboratory may have undermined the UK's whole nuclear deterrent capability.

A member of the US Senate's intelligence committee told the BBC that secret information was passed to Beijing by a Chinese scientist working with the Americans.

John Kyle said the information could enable China to track nuclear submarines.

A new type of radar was being developed at Los Alamos

But on Friday Mr Spellar said: "We are not aware of any unauthorised release of nuclear warhead information of UK origin or of any information which could help others detect our Trident nuclear submarines."

At the daily Ministry of Defence briefing on Kosovo, he said the government had yet to get "a clear read-out" of what the US Senator was saying, but that it was checking as quickly as possible.

The chairman of the Defence Select Committee, Bruce George, is seeking a meeting with the Defence Secretary to find out how harmful such a leak could be.

A worker at the Los Alamos laboratory has admitted transferring 1,000 top-secret files onto an unclassified computer, where they are believed to have been accessed by the Chinese.

It was first thought that the only submarines to be affected were American.

But Mr Kyle said that another nuclear scientist handed over technology to the Chinese which could allow British and American nuclear submarines to be detected.

China has denied stealing nuclear secrets

The news came on the same day that the New York Times quoted US intelligence officials as saying that China is preparing to deploy missiles with a nuclear warhead designed from stolen US secrets.

Pentagon sources told the New York Times that China is expected to deploy Dong Feng-31 missiles, equipped with a small warhead similar to the US neutron bomb on mobile launch systems.

The disclosure came as an about-turn for the US Government, which has until now said there is no evidence that China has deployed nuclear weapons using secret information stolen from US nuclear laboratories.

John Andrew: "This blows twenty years of research by American and British scientists"

The Chinese may have gained information about British nuclear weapons from the so-called legacy codes, secret computer files stored at Los Alamos nuclear laboratory.

They were downloaded onto an unclassified computer by Wen Ho Lee, a Chinese American.

Secret codes 'accessed'

It is reported that details of British nuclear tests were contained in the codes and that they were accessed several times while on the unclassified computer. It is assumed in the US to have been done by Chinese intelligence.

But for Britain, the most worrying aspect of the spy scandals rocking America concerns a Chinese-born physicist called Peter Lee.

He was working on a secret British-American project to develop a new type of radar capable of detecting submarines from the air.

The two governments have spent 20 years carrying out the research, but in May 1997 Mr Lee passed on the information to Chinese scientists.

Britain is particularly vulnerable because the effectiveness of her nuclear deterrence depends on her Trident nuclear submarines remaining undetected.

Chinese officials deny that they have stolen US nuclear secrets.